This topic has been discussed at great length in other threads -- if you're interested, you can uncover a lot through a tag search. That said, I do want to respond. This is just my opinion, obviously, but I base it both on personal experience and on reading many people's stories.
Thinking of a triad as something you could see yourself participating in, even something you'd see as an awesome prospect, is absolutely fine. But if it's your ultimate, first-place, gold-prize ideal and anything else would be falling short, or heavens forfend if it's the only thing you're willing to accept in any new entanglements, you're setting yourselves and anyone you get involved with up for failure, which is neither cool, wise, nor kind.
Why should this be the case?
Think about the times you've fallen in love, or, if you have relatively little experience, think about your friends' stories of how they fell in love. Love is a surprise. It's different each time it happens. Sometimes it comes on like an overwhelming storm, sometimes it's a slow burn from like to very-like to... wait, could it be love? Sometimes it seems perfect but then fizzles quickly, and other times it never actually feels quite right yet you get wrapped up for a time regardless. Sometimes the person who seems like a great match doesn't click with you at all and the person you never expected to even like turns out to rock your world.
Now in light of these facts about love, does it seem at all likely that someone would fall in love with two different people at the same time, at the same rate, to the same degree of intensity, and that those two people should also happen to be in a relationship with each other? Frankly, no. So what happens when the person you and your partner were supposed to be "sharing" falls hard for one of you and not the other? Or falls for you both, but one of you doesn't feel it back? Well, someone is going to be feeling rejected, left behind, maybe even resentful, maybe even heartbroken.
Now think about the breakups you've been through or heard about, the failed relationships, the false starts. The last thing you want is to be reminded of thr person with whom it didn't work out, much less to have to be around them. But what if your partner is still dating that very person? What to do? How can you escape from this painful situation? Sometimes an aborted triad can successfully rearrange itself into a functional vee but more often than not something has to give. Usually it's the newer relationship, with its more tenuous bonds, that gives way and the "unicorn" is cut off entirely, and sometimes it's the preexisting relationship that doesn't make it.
Soooooo, if triads are prone to becoming such messes, how can it be considered advisable to entertain them as a possibility at all?? The key is expectations. If you *expect* a new person to get with you both and for two new romantic relationships to form, which is the definition of unicorn hunting, when in fact this is possible but very unlikely, you are setting the stage for disappointment at the very least and a great deal of pain at the worst. On the other hand if you do NOT expect this to happen but are open to it, well, you can let whatever authentic relationships are there to be had actually form as they naturally want to, rather than trying to shoehorn them into a narrow and difficult pre-set mold. And if a triad by some chance does happen, well... serendipity does occasionally grace one's life. It's just not something it makes senses to bank on, y' know?
This is not even getting into some of the highly problematic trends that seem to often wind up recurring in unicorn-hunting-style triads, due largely to the power imbalance between an established couple and a new person, this is JUST to point out the basic problem of the premise.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.